In general, a gas discharge lamp comprises a cylindrical transparent container, indicated hereinafter as “tube”, with two electrodes at the opposite ends of the tube and a special gas filling (typically comprising mercury vapour) in the tube. The lamp is operated by applying a lamp voltage to the electrodes, resulting in a discharge current in the lamp, which current results in UV light being generated. To produce visible light, the inner surface of the tube, typically glass, is coated with a fluorescent material that converts UV light into visible wavelength. Since gas discharge lamps are known per se, a further explanation is not needed; by way of example, the well-known TL lamp is mentioned.
Although lamps may be operated with different supply voltages, the lamps are typically intended to be supplied with mains voltage. In Europe, mains voltage has a typical rating of 230 V AC at 50 Hz, but in different countries the circumstances may be different. A gas discharge lamp cannot be connected to the mains directly, in view of the fact that a gas discharge lamp has a negative impedance characteristic. Therefore, a gas discharge lamp is always provided with a ballast circuit connected in series with the lamp, the ballast circuit at least comprising an inductor. On the basis of the impedance characteristics of the ballast, a distinction can be made between two types of ballast circuits. A first type of ballast circuit has an inductive impedance; such ballast circuit will also be indicated as an “L-ballast”, and typically consists of an inductor alone. A second type of ballast circuit has a capacitive impedance; such ballast circuit will also be indicated as a “C-ballast”, and typically consists of a series arrangement of an inductor and a capacitor, wherein the impedance of the capacitor (at the mains frequency) is higher than, typically twice as high as, the impedance of the inductor. Many armatures accommodate two lamps, one being provided with an L-ballast and the other being provided with a C-ballast; in such case, which will be indicated as a duo armature, the armature as a whole may exhibit a resistive impedance such that the current provided by the mains is in phase with the mains voltage.
It is noted that, in theory, it is possible to vary the inductance of the inductor in an L-ballast or a C-ballast and/or to vary the capacitance of the capacitor in a C-ballast. However, for standard lamp types also the inductor and capacitor typically have standard values. For instance, in a 36 W TL-lamp, the inductor has a standard value of 0.8 H and the capacitor has a standard value of 3.4 μF.
A particular situation exists when the lamp is OFF and needs to be started. Because of the lamp being OFF, it provides a high impedance at the mains frequency, and the mains voltage is insufficient to cause ignition of the lamp. To solve this problem, the lamp is conventionally equipped with a starter element arranged in parallel to the lamp. This starter element is basically a switch that is closed (conductive) when the lamp is OFF. When the lamp is to be started, the closed starter switch short-circuits the lamp, and the impedance of the lamp circuit is determined mainly by the impedance of the ballast. As a result, a preheat current flows in the lamp circuit, heating the electrodes. After a predetermined preheat time, the starter switch opens, which causes a voltage peak over the heated lamp electrodes sufficient to obtain a discharge to ignite the lamp.
In a more recent development, said conventional starter switch, also indicated as glow starter, has been replaced by an electronic switch.